1. Field of the Invention
The present invention relates to a control system for an engine with a mechanical supercharger and, in particular, to a control system for an engine with a mechanical supercharger in which an overlapping period of an intake valve opening period and an exhaust valve opening period is controlled so as to be changed.
2. Description of Related Art
There is known a conventional control system for an engine with a mechanical supercharger which controls an intake valve and an exhaust valve so as to change an overlapping period of the intake valve opening period and the exhaust valve opening period. Japanese Patent Laid-Open Publication No. 2-119641 discloses such a conventional control system for an engine including a mechanical supercharger, which is driven by the engine and supercharges intake air, and a controller by which an overlapping period of an intake valve and an exhaust valve is changed according to engine operating conditions, i.e., the higher the load of the engine becomes or the higher the engine speed becomes, the longer the overlapping period is provided.
In an engine with a supercharger, the intake air pressure becomes larger than the exhaust air pressure in many cases. On the other hand, such a phenomenon does not occur in an engine with a turbocharger which charges the intake air by utilizing exhaust air energy. Therefore, in an engine with a mechanical supercharger, when the overlapping period of the intake valve and the exhaust valve is constant, the overlapping period constitutes a larger portion of the total period in the low engine speed region than in the high engine speed region. As a result, in an engine with a mechanical supercharger, part of the supplied fuel in an intake port flows out directly to an exhaust port through a combustion chamber of the engine without the fuel being burned. This is called blow-by of fuel.
When the overlapping period of the intake valve and the exhaust valve in the low speed region of the engine is provided as a small value, as in the conventional engine shown in the Patent Publication mentioned above, the blow-by of fuel can be prevented to a certain extent. Further, since the overlapping period can be made large in the high speed region of engine operation, a sufficient absolute overlapping period can be obtained. As a result, remaining burned gas in the combustion chamber is cleaned out by new air so that volumetric efficiency of the intake air in the combustion chamber can be increased. Further, the intake air can be filled by the low charged pressure and therefore the mechanical supercharger has a low load and fuel consumption can be improved.
However, as a matter of fact, it is impossible to prevent effectively the blow-by of fuel to the exhaust port. Namely, when the piston moves from the bottom dead center to the top dead center after the fuel is supplied into the combustion chamber by an intake stroke, the fuel is forced to return back to the intake port through the intake valve while the intake valve is being opened. The returned fuel in the intake port then goes to the exhaust port without being burned when the intake valve opens and the opening period of the intake valve overlaps with that of the exhaust valve. As a result, it is actually impossible to prevent the blow-by of fuel.